Cancer is the general name for a group of more than 100
diseases. Although there are many kinds of cancer, all cancers start because
abnormal cells grow out of control. Untreated cancers can cause serious illness
and death. Cancer can start almost
anywhere in the human body, which is made up of trillions of cells. Normally,
human cells grow and divide to form new cells as the body needs them. When
cells grow old or become damaged, they die, and new cells take their place.
When cancer develops, however, this orderly process
breaks down. As cells become more and more abnormal, old or damaged cells
survive when they should die, and new cells form when they are not needed.
These extra cells can divide without stopping and may form growths called
tumors.
Many cancers form solid tumors, which are masses of
tissue. Cancers of the blood, such as leukemias, generally do not form solid
tumors.
Cancerous tumors are malignant, which means they can
spread into, or invade, nearby tissues. In addition, as these tumors grow, some
cancer cells can break off and travel to distant places in the body through the
blood or the lymph system and form new tumors far from the original tumor.
Unlike malignant tumors, benign tumors do not spread
into, or invade, nearby tissues. Benign tumors can sometimes be quite large,
however. When removed, they usually don’t grow back, whereas malignant tumors
sometimes do. Unlike most benign tumors elsewhere in the body, benign brain
tumors can be life threatening.